Time resolving or demodulating pixels have different fields of applications. Two typical examples are three dimensional (3D) range imaging and fluorescence lifetime imaging. Basically, in both applications higher in-pixel charge transport speed and higher optical sensitivity lead to more accurate per-pixel measurements.
The time-domain demodulation of modulated light signals at the pixel level requires in all approaches the switching of a photo-generated charge current. It is possible to handle electron as well as hole currents. The common methods, however, use the photo-generated electron currents due to the higher mobility of electrons in the semiconductor material.
Some pixel architectures do the necessary signal processing based on the photo-current whereas other architectures work in the charge domain directly.
Common to all pixels is the necessary transfer of charges through the photo-sensitive detection region to a subsequent storage area or to a subsequent processing unit. In the case of charge-domain based pixel architectures, the photo-charge is generally transferred to a storage node. In order to demodulate an optical signal, the pixel has to have implemented at least two integration nodes that are accumulating the photo-generated charges during certain time intervals. Another minimum requirement would be the implementation of at least one integration node and having typically at least one other node for dumping charge carriers as well.
Different pixel concepts have been realized in the last few decades. Schwarte, in “Verfahren and Vorrichtung zur Bestimmung der Phasen-und/oder Amplitudeninformation einer elektromagnetischen Welle”, published Mar. 12, 1998, as DE 197 04 496 A1; introduced a demodulation pixel, which transfers the photo-generated charge below a certain number of adjacent poly-silicon gates to discrete accumulation capacitances. Spirig, in U.S. Pat. No. 5,856,667 [SPI99], titled “Apparatus and method for detection of an intensity-modulated radiation field”, disclosed a charge coupled device (CCD) lock-in concept that allows the in-pixel sampling of the impinging light signal with theoretically an arbitrary number of samples. Another very similar pixel concept has been demonstrated by T. Ushinaga et al. in “A QVGA-size CMOS time-of-flight range image sensor with background light charge draining structure”, published in Three-dimensional image capture and applications VII, Proceedings of SPIE, Vol. 6056, pp. 34-41, 2006; where a thick field-oxide layer is used to smear the potential distribution below the demodulation gates.
A common problem of the afore-mentioned pixel approaches is the slowness of the charge transport through the semiconductor material, which decreases significantly the accuracy or quality of the in-pixel demodulation process. In all pixel structures, the limiting factor for the transport speed is the non-perfect linear potential distribution in the semiconductor substrate that is used to transport the charges through the semiconductor in lateral direction.
The first 3-D cameras that are based on highly-integrated demodulation pixels used the CCD lock-in pixels [SPI99]. However, their limitations in speed and in achievable distance measurement accuracy led to a change of the pixel concepts used. Today's most advanced 3-D cameras implement drift field pixels in order to be able to operate at high frequencies so that 3-D imaging with sub-millimeter resolution becomes possible, as described in [BUE06A] B. Büttgen, “Extending Time-of-Flight Optical 3D-Imaging to Extreme Operating Conditions”, Ph.D. thesis, University of Neuchatel, 2006; and [BUE06B] B. Büttgen, F. Lustenberger and P. Seitz, “Demodulation Pixel Based on Static Drift Fields”, IEEE Transactions on Electron Devices, 53(11):2741- 2747, Nov. 2006. Measurements with such cameras have proven the pixel concept and have shown that gigaHertz demodulation becomes possible with comparable pixel pitches and fill factors as achieved with the standard CCD lock-in pixels [BUE06B].
New concepts of pixels have been explored in the last years accelerating the in-pixel transport of the charges by improving the characteristics of the lateral electric drift fields. Seitz, [SEI02] in U.S. Pat. Appl. Publ. No. US 2006/0108611 A1, invented the first drift field demodulation device that is based on a very high-resistive poly-silicon gate electrode. It even allows very easily the design of pixels that can generate an arbitrary number of samples. The concept was proven by Buettgen [BUE05A] in U.S. Pat. Appl. Publ. No. US 2008/0239466 A1, who disclosed later another concept of demodulation pixels: the static drift field pixel [BUE06B]. In contrast to the architectures mentioned before, the static drift field pixel clearly separates the detection from the demodulation regions within the pixel. It further shows lower power consumption compared to the drift field demodulation approach of Seitz and, at the same time, it supports fast in-pixel lateral charge transport.
Another pixel concept was presented by Nieuwenhoven [NIE05] in “Novel Standard CMOS Detector using Majority Current for guiding Photo-Generated Electrons towards Detecting Junctions”, Proceedings Symposium IEEE/LEOS Benelux Chapter, 2005. In this pixel architecture a modulated lateral electric drift field is generated by the current of majority carriers within the semiconductor substrate. Minority carriers are generated by the photons and transported to the particular side of the pixel depending on the induced current, or applied drift field consequently.
One major application of demodulation pixels is found in real-time 3-D imaging. By demodulating the optical signal and applying the discrete Fourier analysis on the samples acquired, parameters such as amplitude and phase can be extracted for the frequencies of interest. If the optical signal is sinusoidally modulated, the extraction based on at least three discrete samples will lead to the offset, amplitude and phase information. The phase value corresponds proportionally to the sought distance value. Such a harmonic modulation scheme is often used in real-time 3-D imaging systems having incorporated the demodulation pixels [BUE06A].
The precision of the pixel-wise distance measurement is determined by the speed of the in-pixel transfer of the electrons from the area, where they are generated, to the area, where they are accumulated or post-processed. Thus, the ability of the pixel to sample high modulation frequencies is of high importance to perform distance measurements with high accuracy.